Method of operating tire pressure signalling devices on wheels fitted with pneumatic tires

ABSTRACT

A method for operating devices on wheels having pneumatic tires to signal the tire pressure, i.e. the air pressure in the pneumatic tire, in which a pressure sensor measures the tire pressure at regular intervals, and an analysis circuit compares the tire pressure readings with a reference pressure and, in the event of deviations of the tire pressure reading from the reference pressure which exceed a threshold value Δp, transmits by radio, to an indicating device arranged separately from the wheel, a signal reporting the deviation, such that the device meets its energy needs from an electrical battery. Radio operation is discontinued as long as the pressure measured by the pressure sensor is below a preselected second pressure threshold, in particular a preselected absolute pressure value.

The invention relates to a method for operating devices on wheels havingpneumatic tires to signal the tire pressure. A method of this kind isknown from DE 43 03 583 A1. This document discloses d device having adevice, arranged in the valve of the pneumatic tire, for generating awirelessly transmittable pressure decrease indication signal, having abattery as power source, having a preferably semiconductingpiezoresistive pressure sensor, having an analog-digital converter fordigitizing the pressure signal obtained from the pressure sensor, havinga memory for storing the pressure signal, having a comparator whichcompares the pressure signal with a previously stored comparisonpressure signal, having a transmitter, and having a timing switch whichactivates the device from time to time and otherwise keeps it switchedoff in order to save power. The analog-digital converter, the memory,the comparator, and the timing switch are preferably realized in amodule with a microprocessor which compares the pressure signal suppliedby the pressure sensor with the previously stored comparison pressuresignal, generates a signal if the deviation of the pressure signal fromthe comparison pressure signal exceeds a threshold value, and thenactivates a transmitter which thereupon transmits by radio, to areceiver arranged in the vehicle (in particular in the region of thedashboard) and having an analysis circuit and indicating unit, aninformation element concerning the deviation that has been ascertained;said receiver processes the information element and indicates to thedriver the pneumatic tire in which the air pressure deviatessignificantly from the comparison pressure. The reference pressure ofthe pneumatic tire is stored in stable fashion, as the comparisonpressure, each time the air pressure is checked with the aid of anexternal gauge at a filling station, and is set to its referencepressure by means of a compressed air source. For this purpose, themovement of the valve plunger is automatically monitored, for example bymeans of a magnet attached to the valve plunger which in turn actuates amagnetic switch when the valve plunger moves. When the device detects amovement of the valve plunger, the tire pressure measured thereafter isstored as the comparison pressure (reference pressure). Prior to initialuse in a pneumatic tire, a reference pressure can be defined for andentered into the monitoring device by the manufacturer.

The practical applicability of a device of this kind for tire pressuremonitoring depends on the fact that the device uses so little power thatit is can operate for several years without a battery change. Thetransmitter has the highest power consumption. In the case of the knowndevice, it is activated only if the deviation of the measured tirepressure from the reference value exceeds a predefined threshold value.Otherwise the pressure is measured not continuously but rather only atregular intervals, for example every 3 or 4 seconds, and compared withthe reference value which is an absolute value.

Automobile manufacturers require, for such electronic monitoring deviceswhich are preferably provided on the valve of vehicle wheels, a servicelife of at least 5 years without battery replacement. A service life onthis order can now be approximately attained with modem batteries andthe use of integrated circuits having a power consumption of less than 1microampere. Difficulties result, however, if the electronic tirepressure monitoring devices sit in storage for some time before they areused on a vehicle. The storage time can range from a few weeks to twoyears, in particular in the accessories and spare-parts market. Sincethe electronic tire pressure monitoring device nevertheless becomesactive as soon as the battery is inserted by the manufacturer, themaximum possible operating time on the vehicle, as determined by thebattery service life, is shortened by a time equivalent to the storagetime.

It is the object of the present invention to counteract this shorteningof the operating time of the tire pressure monitoring device on thevehicle without impairing its operating reliability.

This object is achieved by means of the present invention. Advantageousdevelopments of the invention are the subject of the dependent claims.

According to the invention, the tire pressure monitoring device isoperated with two different pressure threshold values: As in theexisting art, a first threshold value Δp is defined for the deviation ofthe pressure measured by the pressure sensor from the referencepressure; if the threshold value Δp is exceeded, the transmitter of thetire pressure monitoring device is activated, and the measured pressurevalue or its deviation from the reference value is transmitted by radioto the central analysis and/or indicating device provided in thevehicle. Deviations which do not exceed the first threshold value Δp arecategorized as being negligible, and are not signaled. According to theinvention, a second pressure threshold, in particular an absolutepressure threshold, is also provided; if the measured pressure is so lowthat it falls below said second pressure threshold, this is categorizedas a "wheel not in use" state, and radio operation is discontinued untilthe pressure measured by the pressure sensor exceeds the second pressurethreshold in the opposite direction. If an absolute pressure is selectedas the second pressure threshold, it must be set so low that the changesin pressure which occur during normal operation of the tire pressuremonitoring device--in particular while the vehicle is in motion but alsowhen it is stationery, and both "creeping" pressure losses whichgradually impair vehicle safety and rapid pressure losses--are signaledin all cases before radio operation is discontinued. The preselectedabsolute pressure below which radio operation mode is discontinued musttherefore maintain a much greater distance than Δp from the referencepressure which is intended to be present in the pneumatic tires duringnormal vehicle operation.

The preselected absolute pressure can, for example, be set to a valuewhich is somewhat higher than the atmospheric pressures which usuallyoccur. In this case the tire pressure monitoring device does not resumeradio operation as long as it is not installed in a wheel and thelatter's pneumatic tire is not pumped up. Preferably, however, thepreselected absolute pressure below which radio operation isdiscontinued is set much higher, preferably to a value of 1 to 1.2 barabove atmospheric pressure. Selecting the second pressure threshold inthis fashion on the one hand yields a pressure threshold which isappropriate for practically all pneumatic tires on automobiles and liessufficiently below the reference pressure for normal vehicle operation;on the other hand, there is the advantage that during the periods ofnon-use which occur in the summer for winter tires and in the winter forsummer tires, radio operation can be deliberately discontinued bydecreasing the air pressure in the tire to less than the preselectedabsolute pressure. Sufficient air pressure remains in the tire to allowthe wheels to be stored properly during the period of non-use (over thewinter for summer tires, and over the summer for winter tires). When thetires are then changed again with the change in season, the tirepressure in the wheels to be mounted on the vehicle must then in anycase be reset, so that the tire pressure once again exceeds thepreselected absolute pressure and radio operation can again be resumedin order to signal pressure deviations which exceed the threshold valueΔp.

Power can thus also be economized even interim storage periods; and whensummer tires and winter tires are used alternately, the battery servicelife can be extended beyond the required 5 years, and in somecircumstances can be doubled.

The invention is advantageous in particular with a manner of operationin which the tire pressure monitoring device transmits a signal by radionot only if the deviation of the tire pressure from the referencepressure exceeds the predefined threshold value Δp, but moreover alsotransmits a signal by radio, at regular intervals (for example, 1-minuteintervals), to the central analysis and indicating device in order toinform the latter that it is still ready to operate: this mode ofoperation ensures that malfunctions of the tire pressure monitoringdevice can also be detected, in particular a total failure. The radiosignals which indicate operational readiness make a substantialcontribution to energy consumption, however, and it is thereforeparticularly advantageous if, according to the invention, radiotransmission of the operational readiness signals can be discontinuedduring periods of storage.

The alternative possibility, of switching tire pressure monitoringdevices on or off as necessary by means of a separate switch in thecurrent path between battery and electronics, has the disadvantage thata fault source is thereby introduced which diminishes operationalreliability, especially under the severe service conditions on thevehicle wheel (temperature changes, load cycles, vibration, highcentrifugal accelerations). The method according to the invention, onthe other hand, has the advantage that it requires absolutely norefashioning of the tire pressure monitoring device.

In an advantageous operating mode, an absolute reference value for theair pressure is not stored in the tire pressure monitoring device as thecomparison pressure signal; accordingly, the deviation of the existingtire pressure from the reference value is also not ascertained andsignaled, but rather a drift of the tire pressure is sensed andsignaled, and the comparison pressure signal is continuously adapted forthat purpose. In the simplest case, the existing measured pressure isstored as the comparison pressure, and at the next pressure measurementthe pressure which then exists is compared with the previously storedcomparison pressure. In order to compensate for random measurementerrors and avoid unnecessary activation of the transmitter, however, itis advantageous to constitute the comparison pressure signal not fromthe last pressure signal previously supplied by the pressure sensor, butfrom multiple pressure signals previously supplied by the sensor, e.g.by averaging based on the last three respective measured and storedpressure signals.

In order to make possible an operating principle of this kind, themonitoring device preferably contains a microprocessor or an applicationspecific integrated circuit (ASIC).

This particular operating principle has advantages:

Changes in the tire pressure which occur so slowly that they areharmless do not cause an activation of the transmitter in each case.This applies in particular to pressure changes which are caused bytemperature changes. These changes proceed so slowly that despite a lowthreshold value for the pressure deviation (at present, a pressurethreshold between 10 and 100 mbar is possible with tolerable effort) anda periodic pressure measurement at time intervals which are optimallybetween 1 second and 10 seconds, temperature-related pressure changes donot cause activation of the transmitter even without the need fortemperature compensation of the pressure measurement. This simplifiesthe configuration of the monitoring device in the valve, and spares thebattery;

A drift resulting from unavoidable outward diffusion of air from thetire, which is by far the most frequent reason why air must be added toa tire, also proceeds so slowly that it does not lead to activation ofthe transmitter. The situation is different in the existing art: there,if the tire pressure has decreased, as a result of gradual andcontinuous diffusion, to the point that the pressure loss exceeds thethreshold value, the transmitter is activated at each subsequentmeasurement which takes place, for example, every four seconds, becausethe value has fallen below the threshold: this continues until thedriver has visited a filling station and corrected the tire pressure.Until then, however, a great deal of power is consumed from the batteryfor transmission. According to the invention, however, a slow drift inthe tire pressure caused by diffusion does not cause activation of thetransmitter. A hazardous situation is nevertheless not created thereby,since such slow pressure losses can be identified and corrected by meansof checks occurring at longer intervals, for example at each refuelingstop. Advantageously, however, the check is not entrusted to the driveron the occasion of a refueling stop; instead a report of the existingtire pressure is made--at time intervals which are long in terms of thetime intervals at which the tire pressure is regularly checked (1 secondto 10 seconds), and regardless of whether or not the threshold value hasbeen exceeded--to the receiver provided at a central point in thevehicle, in which the signaled pressure values are processed andevaluated. A slow drift in the tire pressure is thereby sensed atreasonable, longer time intervals which spare the battery in the valve,and is indicated to the driver by means of, for example, an advisorysaying in effect, "Please increase the right front tire pressure at thenext refueling stop." Suitable time intervals at which an informationelement regarding the tire pressure is sent in all cases, regardless ofwhether a predefined threshold value has been exceeded, are between oneminute and one hour, the longer time intervals being particularlypreferred.

The design of the valve is simplified by the fact that a sensor is notrequired in order to monitor movement of the valve plunger.

Because a comparison with an absolute reference value of the airpressure is not performed in the monitoring device in the valve, it isalso not necessary for a complete analysis to take place at each point:rather only a drift check must take place. Everything else can beentrusted to a central analysis computer. This has the further advantagethat the tire pressure check is more reliable and is independent ofoperating errors when adding air to the tire and of calibration errorsin the air pump gauge. If the comparison with an absolutely predefinedreference pressure occurs only in a central computer, the latter canalso indicate errors in the addition of air and authorize an immediatecorrection; in addition, centralized parameters can be taken intoaccount, e.g. the loading status (cargo weight, particularly in the caseof commercial vehicles; number of persons aboard, in the case of busesand passenger cars).

Sudden pressure losses resulting from damage to the tire or to its valveare particularly hazardous to vehicle safety. Pressure losses of thiskind can be determined with particular reliability because themeasurement intervals (the "first" time intervals) can be short and thethreshold value can be row, without shortening the battery service lifethat is possible in the existing art. In an advantageous development ofthe invention, provision is made for the "first" time intervals to beshortened as soon as a pressure loss exceeding the threshold value Δp isascertained. A single pressure loss reading exceeding the thresholdvalue might be the result of a random measurement error. In order toeliminate random warning signals, it is advantageous, following apressure loss reading which exceeds the threshold value, to clarify thesituation by causing additional measurements to follow at an increasedmeasurement rate. If the subsequent measurements confirm the pressuredrop, a warning signal can be sent very quickly. If the subsequentmeasurements do not confirm the pressure drop, however, it is apparentthat an incorrect measurement was present, and the transmitter will noteven be activated. In this fashion, with no loss of safety, the batteryis spared and the driver is not irritated by incorrect measurements. Ina preferred development of the invention, it is impossible for radiooperation to be discontinued while pressure measurements are being madeat the increased measurement rate, so that hazardous pressure losses,both gradual and rapid, can be detected and signaled in all cases.

Just as a pressure loss exceeding the threshold value can beascertained, so can a pressure rise exceeding the threshold value Δp.This usually involves the operation of adding air. In this case thetransmission of a signal reproducing the tire pressure can be postponeduntil the tire pressure has stabilized, i.e. the addition of air iscomplete.

Once the pressure loss and the pressure rise drop back below thresholdvalue Δp, the time intervals at which the tire pressure is checked bymeans of the monitoring device arranged in the valve are once againlengthened.

In practical terms, the following procedure can be used to carry out theinvention: As long as the pressure measured by the pressure sensor islower than the second pressure threshold (preselected absolutepressure), a pressure measurement is performed, for example, every 60seconds, and a check is made as to whether the measured pressure isstill below said second pressure threshold, which preferably isapproximately 1 bar above atmospheric pressure. A data transfer viaradio is so far not occurring. If, however, a pressure is measured whichis greater than the second pressure threshold, the radio inhibition islifted and the tire pressure monitoring device is in its normaloperating state, in which a comparison between setpoint and actual valueis performed, and impermissibly high deviations are signaled by radio.If the measured pressure falls below the second pressure threshold atany time, radio operation is once again inhibited, except if the tirepressure monitoring device is in the condition of an increasedmeasurement rate, which as a result of a rapid pressure drop ismaintained for a fixed period of, for example, 180 seconds. During thisfixed period in which the system is operating with an increasedmeasurement and radio transmission rate, the hazardous pressure drop canbe signaled; and only after said fixed period of, for example, 180seconds has elapsed is further radio traffic inhibited if the pressurefalls below the second pressure threshold (the predefined absolutepressure) and the hazardous pressure drop has been signaled.

We claim:
 1. A method for operating devices on wheels having pneumatic tires to signal the tire pressure, i.e. the air pressure in the pneumatic tire,in which a pressure sensor measures the tire pressure at regular intervals, and an analysis circuit compares the tire pressure readings with a reference pressure and, in the event of deviations of the tire pressure reading from the reference pressure which exceed a threshold value Δp, transmits by radio, to an indicating device arranged separately from the wheel, a signal reporting the deviation, such that the device derives its energy needs from an electrical battery; wherein radio operation is discontinued as long as the pressure measured by the pressure sensor is below a preselected second pressure threshold.
 2. The method as defined in claim 1, wherein the second pressure threshold is constituted by an absolute pressure.
 3. The method as defined in claim 2, wherein radio operation is discontinued if the tire pressure is less than 1.2 bar above ambient atmospheric pressure.
 4. The method as defined in claim 2, wherein radio operation is discontinued if the tire pressure is less than 1.0 bar above ambient atmospheric pressure.
 5. The method as defined in claim 2, for operating a tire pressure monitoring device whose analysis circuit has an analog-digital converter for digitizing the pressure signal obtained from the pressure sensor, a memory for storing the pressure signal, a comparator, a means radio transmitter, and timing switch which activates the device at the time intervals for a measurement and comparison operation and keeps it switched off at other times in order to save power, wherein the comparator means stores the transmitted pressure signal and, when the deviation of the pressure signal for the comparator pressure signal has been formed, forms with said pressure signal a new comparison pressure signal which replaces the previously stored comparison pressure signal.
 6. The method as defined in claim 1, wherein radio operation is discontinued if the tire pressure is less than 1.2 bar above ambient atmospheric pressure.
 7. The method as defined in claim 1, wherein radio operation is discontinued if the tire pressure is less than 1.0 bar above ambient atmospheric pressure.
 8. The method as defined in claim 1, for operating a tire pressure monitoring device whose analysis circuit has an analog-digital converter for digitizing the pressure signal obtained from the pressure sensor, a memory for storing the pressure signal, a comparators means a radio transmitter, and a timing switch which activates the device at the time intervals for a measurement and comparison operation and keeps it switched off at other times in order to save power, wherein the comparator means stores the pressure signal and, when the deviation of the pressure signal from a comparison pressure signal has been formed, forms with said pressure signal a new comparison pressure signal which replaces the previously stored comparison pressure signal.
 9. The method as defined in claim 8, wherein the new comparison pressure signal is formed from multiple pressure signals previously supplied by the pressure sensor.
 10. The method as defined in claim 9, wherein the new comparison pressure signal is formed by averaging from the multiple pressure signals supplied by the pressure sensor and previously stored.
 11. The method as defined in claim 8, wherein the timing switch activates the device at "first" time intervals of between 1 second and 10 seconds.
 12. The method as defined in claim 11, wherein the timing switch activates the transmitter at "second" time intervals which are long compared with the "first" time intervals, and causes a status signal to be emitted even if a deviation exceeding the threshold value Δp has not been ascertained.
 13. The method as defined in claim 12, wherein the "second" time intervals are between 1 minute and 1 hour.
 14. The method as defined in claim 11, wherein the threshold value Δp and the "first" time intervals at which the device is activated are adjusted to one another so that the quotient of the threshold value Δp and the "first" time interval is large compared with rates of change in tire pressure which typically occur, in the absence of leaks in the tire or the valve, due to changes in tire temperature as a result of vehicle operation.
 15. The method as defined in claim 11, wherein a gradual pressure loss which lies below threshold value Δp is determined by analysis of the status signals (tire pressure information elements) sent at the "second" time intervals.
 16. The method as defined in claim 11, wherein if a deviation exceeding the threshold value Δp is ascertained, the "first" time intervals during which the pressure measurement and comparison operations take place in the device are shortened, and the transmitter is activated if the fact that the threshold value Δp has been exceeded is confirmed for a number of pressure measurement and comparison operations which follow during the shortened "first" time intervals.
 17. The method as defined in claim 16, wherein if the pressure loss or pressure rise drops back below the threshold value Δp, the shortened "first" time intervals are lengthened again to the original "first" time intervals and only then is a new comparison pressure signal formed.
 18. The method as defined in claim 8, wherein the absolute tire pressure values are processed and evaluated in the receiving and indicating device, whereas in the device arranged on the wheel, the measured tire pressure values are evaluated only with regard to a drift in the tire pressure.
 19. The method as defined in claim 1, wherein the absolute tire pressure values are processed and evaluated in the receiving and indicating device, whereas in the device arranged on the wheel, the measured tire pressure values are evaluated only with regard to a drift in the tire pressure.
 20. The method as defined in claim 1, for operating a tire pressure monitoring device whose analysis circuit has an analog-digital converter for digitizing the pressure signal obtained from the pressure sensor, a memory for storing the pressure signal, a comparator, means a radio transmitter, and timing switch which activates the device at the time intervals for a measurement and comparison operation and keeps it switched off at other times in order to save power, wherein the comparator means stores the transmitted pressure signal and, when the deviation of the pressure signal for the comparator pressure signal has been formed, forms with said pressure signal a new comparison pressure signal which replaces the previously stored comparison pressure signal. 